TestAvAudioEngineAppViewController.cs

using Foundation;
using System;
using System.CodeDom.Compiler;
using UIKit;
using AudioToolbox;
using System.Runtime.InteropServices;
using AVFoundation;
using System.Linq;



namespace TestAvAudioEngineApp
{
	partial class TestAvAudioEngineAppViewController : UIViewController
	{
		public static uint kOutputBus = 0;
		public static uint kInputBus = 1;

		AVAudioEngine audioEngine;
		AVAudioInputNode audioInputNode;
		AVAudioMixerNode mixerNode;

		public TestAvAudioEngineAppViewController (IntPtr handle) : base (handle)
		{
		}

		public override void ViewDidLoad ()
		{
			base.ViewDidLoad ();

			//create main block...implictely creates an output
			audioEngine = new AVAudioEngine ();

			var playerNode = new AVAudioPlayerNode ();

			//get microphone input from engine node
			audioInputNode = audioEngine.InputNode;
			//get mixer node from engine
			mixerNode = audioEngine.MainMixerNode;
			//connect the chain

			var inputFormat = audioInputNode.GetBusInputFormat (0);
			var inChannels = inputFormat.ChannelCount;
			var outputFormat = playerNode.GetBusOutputFormat (0);
			var outChannels = outputFormat.ChannelCount;
			var sampleRate = inputFormat.SampleRate;

			Console.WriteLine ("Input format: {0}", inputFormat);
			Console.WriteLine ("Output format: {0}", outputFormat);

			var channels = inChannels;
			int counter = 0;

			audioInputNode.InstallTapOnBus (0, 4096, inputFormat, delegate(AVAudioPcmBuffer inBuffer, AVAudioTime when) {
				var outBuffer = new AVAudioPcmBuffer (inputFormat, inBuffer.FrameCapacity);
				outBuffer.FrameLength = inBuffer.FrameLength;

				var sr = inputFormat.SampleRate;

				unsafe {
					var outDataPointers = new float*[channels];
					var inDataPointers = new float*[channels];
					for (int i = 0; i < channels; i++) {
						// buffer.FloatChannelData is a native array of pointers to audio data.
						// convert that into a managed array of pointers to audio data.
						outDataPointers [i] = (float *) Marshal.ReadIntPtr (outBuffer.FloatChannelData, i * IntPtr.Size);
						inDataPointers [i] = (float *) Marshal.ReadIntPtr (inBuffer.FloatChannelData, i * IntPtr.Size);
					}

					for (int i = 0; i < channels; i++) {
						// get the pointer to the audio data for the channel we're processing.
						var outptr = outDataPointers [i];
						var inptr = inDataPointers [i];
						for (int j = 0; j < outBuffer.FrameLength; j++) {
							var val = (float) Math.Sin(441.0*j*2*Math.PI/sampleRate) / 10f;
							// mix sine wave with mic input.
							*outptr++ = val + *inptr++;
						}
					}  
				}
				// schedule the audio data we've created
				playerNode.ScheduleBuffer (outBuffer, () => {
				});
			});

			audioEngine.AttachNode (playerNode);
			audioEngine.Connect (playerNode, mixerNode, inputFormat);

			NSError error;
			audioEngine.StartAndReturnError (out error);

			playerNode.Play ();
		}

	}
}

I'm surprised you can allocate:

var outBuffer = new AVAudioPcmBuffer (inputFormat, inBuffer.FrameCapacity);

within the block. Seems like you should allocate that outside the block...